The present invention relates to the melt extrusion of a thermoplastic polymer.
The melt extrusion of a thermoplastic polymer to form fibers and nonwoven webs generally involves forcing a molten polymer through a plurality of orifices to form a plurality of molten threadlines, contacting the molten threadlines with a fluid, usually air, directed so as to form filaments or fibers and attenuate them. The attenuated filaments or fibers then are randomly deposited on a surface to form a nonwoven web.
The more common and well known processes utilized for the preparation of nonwoven webs are meltblowing, coforming, and spunbonding.
Meltblowing references include, by way of example, U.S. Pat. No. 3,016,599 to Perry, Jr., U.S. Pat. No. 3,704,198 to Prentice, U.S. Pat. No. 3,755,527 to Keller et al., U.S. Pat. No.3,849,241 to Butin et al., U.S. Pat. No.3,978,185 to Butin et al., and U.S. Pat. No. 4,663,220 to Wisneski et al., See, also, V. A. Wente, xe2x80x9cSuperfine Thermoplastic Fibersxe2x80x9d, Industrial and Engineering Chemistry, Vol. 48, No. 8, pp. 1342-1346 (1956); V. A. Wente et al., xe2x80x9cManufacture of Superfine Organic Fibersxe2x80x9d, Navy Research Laboratory, Washington, D.C., NRL Report 4364 (111437), dated May 25, 1954, United States Department of Commerce, Office of Technical Services; and Robert R. Butin and Dwight T. Lohkamp, xe2x80x9cMelt Blowingxe2x80x94A One-Step Web Process for New Nonwoven Productsxe2x80x9d, Journal of the Technical Association of the Pulp and Paper Industry, Vol. 56, No.4, pp. 74-77 (1973).
Coforming references (i.e., references disclosing a meltblowing process in which fibers or particles are commingled with the meltblown fibers as they are formed) include U.S. Pat. No. 4,100,324 to Anderson et al. and U.S. Pat. No. 4,118,531 to Hauser.
Finally, spunbonding references include, among others, U.S. Pat. No. 3,341,394 to Kinney, U.S. Pat. No. 3,655,862 to Dorschner et al., U.S. Pat. No. 3,692,618 to Dorschner et al., U.S. Pat. No. 3,705,068 to Dobo et al., U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. No. 3,853,651 to Porte, U.S. Pat. No. 4,064,605 to Akiyama et al., U.S. Pat. No. 4,091,140 to Harmon, U.S. Pat. No. 4,100,319 to Schwartz, U.S. Pat. No. 4,340,563 to Appel and Morman, U.S. Pat. No. 4,405,297 to Appel and Morman, U.S. Pat. No. 4,434,204 to Hartman et al., U.S. Pat. No. 4,627,811 to Greiser and Wagner, and U.S. Pat. No. 4,644,045 to Fowells.
Some of the difficulties or problems routinely encountered with melt extrusion processes are, by way of illustration only, thermal degradation of the polymer, plugging of extrusion dies, and limitations on fiber diameters, throughput, and production rates or line speeds. Fiber diameters generally are a function of the diameter of the orifices through which the polymer is extruded, although the temperature and velocity of the attenuating fluid can have a significant effect. For some applications, fiber diameters of less than about 10 micrometers are desired. Throughput primarily is a function of the melt flow rate of the polymer, while production rates depend in large measure upon throughput. In other words, throughput and production rates generally are dependent upon the viscosity of the molten polymer being extruded. The difficulties and problems just described result largely from efforts to manipulate melt viscosity to achieve desired throughput and/or production rates. Accordingly, there are opportunities for improvements in melt extrusion processes based on improved melt viscosity control.
The present invention addresses some of the difficulties and problems discussed above by providing an apparatus and a method for the melt extrusion of a thermoplastic polymer, e.g., as fibers and nonwoven webs, which apparatus and method utilize ultrasonic energy to assist in the melt-extrusion process. The apparatus includes a die housing and a means for applying ultrasonic energy to a portion of the molten thermoplastic polymer. The die housing defines a chamber adapted to receive the molten thermoplastic polymer an inlet orifice adapted to supply the chamber with the molten thermoplastic polymer, and an extrusion orifice adapted to receive the molten thermoplastic polymer from the chamber and extrude the polymer. The means for applying ultrasonic energy is located within the chamber.
In one aspect of the present invention, the die housing has a first end and a second end and the extrusion orifice is adapted to receive the molten thermoplastic polymer from the chamber and extrude the polymer along a first axis. The means for applying ultrasonic energy to a portion of the molten thermoplastic polymer is an ultrasonic horn having a first end and a second end. The horn is adapted, upon excitation by ultrasonic energy, to have a node and a longitudinal mechanical excitation axis. The horn is located in the second end of the die housing in a manner such that the first end of the horn is located outside of the die housing and the second end is located inside the die housing, within the chamber, and is in close proximity to the extrusion orifice.
The molten thermoplastic polymer may be extruded as, by way of example, a fiber. In such case, the longitudinal excitation axis of the ultrasonic horn desirably will be substantially parallel with the first axis. Furthermore, the second end of the horn desirably will have a cross-sectional area approximately the same as or less than a minimum area which encompasses all extrusion orifices in the die housing.
The present invention contemplates the use of an ultrasonic horn having a vibrator means coupled to the first end of the horn. Typically, the vibrator means will be a piezoelectric transducer. The transducer may be coupled directly to the horn or by means of an elongated waveguide. The elongated waveguide may have any desired input:output mechanical excitation ratio, although ratios of 1:1 and 1.5:1 are typical for many applications. The ultrasonic energy typically will have a frequency of from about 18 kHz to about 60 kHz.
The present invention also contemplates a method of forming a fiber. The method involves supplying a molten thermoplastic polymer and extruding the polymer through an extrusion orifice in a die assembly to form a threadline. The die assembly will be a die housing and a means for applying ultrasonic energy to a portion of the molten thermoplastic polymer as already defined. The means for applying ultrasonic energy is at least partially surrounded by the molten thermoplastic polymer and is adapted to apply the ultrasonic energy to the molten thermoplastic polymer as it passes into the extrusion orifice. While extruding the molten thermoplastic polymer, the means for applying ultrasonic energy is excited with ultrasonic energy. The threadline which emerges from the extrusion orifice then is attenuated to form a fiber.
The present invention further contemplates a method of forming from a thermoplastic polymer a fiber having entrapped along the length thereof bubbles of a gas. This method also involves supplying a molten thermoplastic polymer and extruding the polymer through an extrusion orifice in a die assembly to form a threadline. The die assembly may be a die assembly and an ultrasonic horn for applying ultrasonic energy to a portion of the molten thermoplastic polymer as already defined. While extruding the molten thermoplastic polymer, the ultrasonic horn is excited with ultrasonic energy under conditions sufficient to maintain cavitation. The threadline which emerges from the extrusion orifice then is attenuated to form a fiber.
Cavitation results in the formation of bubbles of a gas within the threadline, which bubbles remain entrapped. Attenuation to form a fiber elongates, but does not destroy, the bubbles. Because of the presence of the bubbles, the density of the fiber is less than that of an otherwise identical fiber lacking the entrapped bubbles of gas. As an example, the density of a fiber containing bubbles of a gas may be less than about 90 percent of the density of an otherwise identical fiber lacking the entrapped bubbles of gas. As another example, the density of the fiber may be in a range of from about 20 to about 90 percent of the density of an otherwise identical fiber lacking the entrapped bubbles of gas.